1. Field of the Invention
The present invention relates to a LED lamp configured to be lit up by application of an input current varying in response to a dimming operation with use of a phase control dimmer or so forth.
2. Background Art
In comparison with a well-known incandescent lamp (including a halogen lamp), a light emitting diode (hereinafter referred to as “LED”) is more advantageous in its lower power consumption and longer lifetime. Therefore, with growing awareness of demanders for ecology, the LED has been rapidly prevailed as a countermeasure for energy saving in a wide range of usage. Moreover, needs to use the LED instead of the incandescent lamp have been also increased. In addition, brightness of a light from the LED has been generally varied by a dimming operation with use of a phase control dimmer or so forth.
Incidentally, the well-known incandescent lamp is configured to gradually dim a light by reducing the conduction angle of a commercial alternating-current voltage through a dimming operation with use of a phase control dimmer. When the incandescent lamp is gradually dimmed, the temperature of a filament configured to emit a light inside the incandescent lamp is lowered. Accordingly, the color temperature of a light to be emitted from the filament is also lowered (gets reddish).
By contrast, the color temperature of a light to be emitted from an LED is not varied even when the LED is dimmed by reducing a current. Therefore, it has been demanded to produce an LED lamp configured to vary its color temperature in response to a dimming operation as with the well-known incandescent lamp, whereby a desired atmosphere can be maintained in a guest room. This is because such LED lamps can be used as alternatives for halogen lamps to be used in hotels and so forth in which chances of dimming are relatively high.
Publication of Japanese translation of PCT international application No. JP-A-2008-507817 can be herein exemplified as a well-known technology of an LED driver circuit whereby a color temperature is varied. The LED driver circuit described in the Publication No. JP-A-2008-507817 includes lamp ballast circuits and a ballast control circuit. The lamp ballast circuits are respectively configured to supply a power to at least two LEDs that the color temperatures thereof are different from each other. The ballast control circuit is configured to output a lamp control signal in accordance with the value of an input supply voltage that varies in response to a dimming operation. Further, the magnitude of a voltage to be supplied from each lamp ballast circuit to its relevant LED lamp is configured to vary in response to the lamp control signal to be supplied from the ballast control circuit.
With reduction in input supply voltage, the LED drive circuit described in the Publication No. JP-A-2008-507817 is configured to reduce the ratio of a voltage to be supplied to an LED with a relatively high color temperature, while being configured to increase the ratio of a voltage to be supplied to an LED with a relatively low color temperature. Accordingly, the color temperature of a light to be emitted from the entire LED lamp can be reduced in accordance with reduction in light intensity just like a light to be emitted from an incandescent lamp.
As described above, in the LED driver circuit described in the Publication No. JP-A-2008-507817, each lamp ballast circuit is configured to supply a power to its relevant LED lamp. In other words, the required number of lamp ballast circuits is the same as the number of the LED lamps relevant to the lamp ballast circuits. This results in increase in circuit size of the LED driver circuit. It has been thus difficult to satisfy the demand for reduction in size of an LED lamp as an alternative of a halogen lamp.
In addition, the LED driver circuit described in the Publication No. JP-A-2008-507817 is of a voltage control type configured to: receive an input supply voltage that varies in response to a dimming operation; rectify the input supply voltage; and control LEDs with different color temperatures in accordance with a square wave of the resultant rectified voltage. Therefore, a drawback has been also produced that a current to be supplied to an LED contains a missing part roughly at a zero voltage and this is likely to be a cause of flicker. To solve such a drawback of flicker, conversion of voltage or current in accordance with a square wave is required for the lamp ballast circuits. Accordingly, another drawback is inevitably produced that the circuit size is increased and the circuit structure is complicated.
The present invention has been developed in view of the aforementioned drawbacks of the well-known art. Therefore, it is a main object of the present invention to provide an LED lamp that is capable of dimming a light and emitting a light with a low color temperature with a simple circuit structure as with an incandescent lamp.